This avoids one pointless page allocation/insertion in case of non-zerora_pages, or many pointless readahead calls in case of zero ra_pages.

Note that if a user sets ra_pages to less than PIPE_BUFFERS=16 pages, he willnot get expected readahead behavior anyway. The splice code works in batchesof 16 pages, which can be taken as another form of synchronous readahead.